Liquid crystal device, driving method thereof, liquid crystal apparatus, and illumination apparatus

ABSTRACT

An illumination apparatus includes a liquid crystal device (L) disposed in the optical path of light projected from a light source (P) and a driving circuit (D) connected to the liquid crystal device for applying a special driving voltage to a composite film (1) of the device (L). The applied driving voltage alternately causes the composite film (1) to attain an opaque state and a transparent state. The time periods of the alternating voltage pulses are varied, thereby changing the ratio of the duration of the opaque state to the duration of the transparent state per unit time period, and thus controlling the power of light transmitted through the composite film (1) without undesirably altering the color spectrum of the transmitted light.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to copending U.S. application Ser. No.08/122,576, which was filed Sep. 28, 1993, is titled "Liquid CrystalDimmer Plate and Lighting System Including the Same", and namesinventors overlapping with the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to liquid crystal devices for adjustingthe transmission of light, a method of driving such devices, andespecially an illuminating apparatus incorporating such a liquid crystaldevice for adjusting illumination light for television or movie filmingand photographing, or for a projecting type television receiver, a slideprojector or the like.

2. Description of the Related Art

A liquid crystal device using materials such as twisted nematic liquidcrystal, super twisted nematic liquid crystal and ferro-electric liquidcrystal needs a polarizer which causes a loss of more than 50% ofunpolarized light. When a liquid crystal device using any of theabove-described materials is used as a light-adjusting element in anillumination apparatus using a high power light source it is inevitablethat the polarizer's temperature will be greatly increased due toabsorbance of light.

In contrast, a polarizer is not needed in a liquid crystal device havinga composite film in which a liquid crystal material fills a series ofpores in a film that forms a transparent matrix having athree-dimensional network structure, or in which a liquid crystalmaterial is dispersed in particles in a film that forms a transparentmatrix sandwiched between transparent substrates having a pair oftransparent conductive films. Therefore, the above-describeddisadvantage regarding heating of the polarizer is overcome.

In the above-described liquid crystal device, when no voltage isapplied, the liquid crystal molecules are in a random state based on theconfiguration of the interface between the liquid crystal molecules andthe transparent matrix, which is known as the anchoring effect. Thus, inthat state incident light is scattered and the composite film is opaque.When a voltage, usually a rectangular wave or a sinusoidal wave of about200 Hz, is applied to the region between the transparent substrateshaving the pair of transparent conductive films with the composite filmheld therebetween, the liquid crystal molecules having positivedielectric anisotropy (Δε) are oriented in the direction of the electricfield and are thereby gradually ordered in a manner increasing lighttransmittance. Thus, an electro-optic effect is brought about, and atransparent state results. Note that the term "transmittance" generallyindicates the ratio of the power of the light emitted from an element,relative to the power of the light incident to the element. However, inthe case of a light-scattering type liquid crystal device, the termindicates the ratio of the power of the light emitted in the range of acertain angle, relative to the power of a collimated light incident tothe element. The angle may be determined depending upon the conditionfor using the element. Herein, light transmitted within the range of theangle is referred to as non-scattered light.

In a conventional liquid crystal device using any materials, however,the dependence of the birefringence of the liquid crystal on wavelengthbasically causes the spectrum of transmitted light to change dependingupon the applied voltage state. Furthermore, the spectrum of transmittedlight largely fluctuates particularly in a middle state between theopaque state and the transparent state.

Also in the above-described liquid crystal device using the compositefilm, the relation between the light transmittance in each wavelengthand the applied voltage is not constant in a middle state between theopaque state and the transparent state. Moreover, the transmittance atthe same voltage greatly fluctuates depending upon the wavelength.Particularly, the light transmittance is greater for longer wavelengthsthan for shorter wavelengths. Therefore the spectrum of transmittedlight greatly fluctuates depending upon the applied voltage.Furthermore, the ratio of transmittance of each wavelength changesdepending upon the applied voltage, and therefore the color tone of thetransmitted light also changes as the applied voltage changes.

Through a study for the cause of the above described effects, thefollowing has been discovered. Namely, when a voltage is applied to aliquid crystal device the liquid crystal molecules are oriented in thedirection of the electric field as described above. However, theintensity of the electric field is insufficient in the middle statebetween the opaque state and the transparent state, and therefore theorientation of the liquid crystal molecules is disturbed by the abovedescribed anchoring effect in the vicinity of the interface of theliquid crystal molecules and the transparent matrix. Accordingly, lightof a short wavelength is mainly scattered in a region in the vicinity ofthe interface, the transmittance of short wavelength light is lower thanthe transmittance of long wavelength light, and the transmitted lightgives a spectrum in which the longer wavelengths prevail.

The dependence of the scattering intensity on the wavelength in turndepends upon the degree of disturbance of the orientation of the liquidcrystal molecules, based on the area of the region in which theorientation of the liquid crystal molecules is disturbed, the degree ofdisturbance of the liquid crystal molecules in the above-describedregion or the like. The degree of disturbance of the orientation dependson both the applied voltage and the anchoring effect, and therefore thetransmittance of each wavelength changes depending upon the appliedvoltage, thus changing the color tone of the transmitted light.

Therefore, when the above-described liquid crystal device is used as alight adjusting element, it is impossible to obtain light with itsshorter wavelengths prevailing, or light without any wavelengthdependence i.e. white light, and the color tone of the light cannot bemade constant.

Therefore, the above-described liquid crystal device has already beenreduced to practice for a display with a function of switching betweentwo states, namely opaque and transparent states, but has not yet beenapplied to a light-adjusting element capable of sequentially adjustingits light transmittance though such a device has long been desired.

Furthermore, the conventional liquid crystal device is not capable ofoperating in a high temperature environment of 100° C. or higher.

SUMMARY OF THE INVENTION

The present invention was made as an improvement in view of theabove-described situation. It is an object of the invention to provide amethod of driving a liquid crystal device that will enable thesequential adjustment of the light transmittance of the liquid crystaldevice while keeping the distribution of the spectrum of the transmittedlight from shifting and the color tone of the transmitted light fromabnormally changing. The invention further aims to provide a liquidcrystal apparatus employing such a driving method, an illuminationapparatus incorporating such a liquid crystal apparatus, and a liquidcrystal device capable of operating at a temperature of 100° C. orhigher without changing its characteristic.

A method of driving a liquid crystal device according to the presentinvention solves the above-described problem. The method relates todriving a liquid crystal device in which a composite film is sandwichedbetween transparent substrates having a pair of transparent conductivefilms. The composite film comprises a liquid crystal material that fillsa series of pores in a film that forms a transparent matrix having athree-dimensional network structure or comprises a liquid crystalmaterial that is dispersed in particles in a film that forms atransparent matrix. In the method, the driving voltage applied acrossthe region between the electrodes described above attains a voltagewaveform alternatively switching between first and second voltage statesin which the composite film attains an opaque state and a transparentstate, respectively. The method further involves changing the timeperiod between both voltage states to change the ratio of the timeperiod of the opaque state to the time period of the transparent stateof the composite film per unit time period. In this manner, the power ofnon-scattered light transmitted through the composite film per unit timeperiod is controlled without substantially changing the spectrum.

The liquid crystal apparatus according to the invention includes acomposite film in which a liquid crystal material fills a series ofpores in a film that forms a transparent matrix having athree-dimensional network structure, or in which a liquid crystalmaterial is dispersed in particles in a film that forms a transparentmatrix. The composite film is sandwiched between transparent substrateshaving a pair of transparent conductive films. The apparatus furtherincludes a driving circuit for applying a voltage having a waveformalternately switching between voltage states in which the composite filmattains an opaque state and a transparent state, respectively, and forchanging the time period between both voltage states with an externalsignal to change the ratio of the time period of the opaque state to thetime period of the transparent state of the composite film per unit timeperiod, thereby controlling the power of non-scattered light transmittedthrough the composite film per unit time period.

Further, an illumination apparatus according to the present invention ischaracterized by the arrangement of the liquid crystal device of theliquid crystal apparatus as described above in the optical path of lightprojected from a light source.

Further, as an alternative, the liquid crystal device of the apparatusaccording to the present invention can have generally the structuredescribed above, but wherein only one or at least one of the substratesis a transparent substrate having a transparent conductive film. Thisembodiment is characterized in that the liquid crystal material shows aliquid crystal phase and changes from the opaque state to thetransparent state in response to a voltage applied across the compositefilm between the above-described two conductive films at a temperatureof at least 100° C. or higher.

According to the present invention implemented by the above-describedstructure, since the power of non-scattered light transmitted throughthe composite film per unit time period is determined by the ratio ofthe time period of the opaque state relative to the time period of thetransparent state per unit time period, controlling or adjusting theratio of the time period of the opaque state relative to the time periodof the transparent state permits a continuous adjustment of the level oflight transmittance through the composite film ranging from the opaquestate to the transparent state. Furthermore, the spectrum distributionof the transmitted light at any selected ratio of the time period of theopaque state relative to the time period of the transparent state perunit time period is substantially identical to the transmittancespectrum in the transparent state. Therefore, there is no possibilitythat the spectrum distribution of the transmitted light shifts or thecolor tone of the transmitted light abnormally changes depending uponthe applied voltage particularly in a middle state between the opaquestate and the transparent state, so that the spectrum distributionbecomes substantially constant.

Further, substantially equalizing the respective spectrum distributionsof the opaque state and the transparent state further reduces thepossibility of a change in the spectrum distribution in the middle statebetween the opaque state and the transparent state. Furthermore, becausethe liquid crystal device according to the invention implemented by theabove-described structure uses liquid crystal material that has a liquidcrystal phase in a temperature range of at least 100° C. or higher anddoes not use a polarizing plate that has a poor heat resistance, itachieves responsiveness from the opaque state to the transparent statein response to an applied voltage in a temperature environment of 100°C. or higher.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view showing the structure of one embodiment of aliquid crystal apparatus according to the invention, and FIG. 1B is aschematic view showing the structure of one embodiment of anillumination apparatus according to the invention incorporating theabove-described liquid crystal apparatus;

FIG. 2 is a sectional view showing one example of the layered structureof the liquid crystal device used for the liquid crystal apparatus inFIG. 1A;

FIGS. 3A and 3B are sectional views respectively showing an enlargeddetail view of two different embodiments of part of the composite filmof the above-described liquid crystal device;

FIG. 4A is a waveform chart showing the waveform of a pulse-shapeddriving voltage applied to a liquid crystal device in a method ofdriving a liquid crystal device according to the invention, and FIG. 4Bis a waveform chart showing the waveform of a driving voltage applied toa liquid crystal device in a conventional driving method;

FIG. 5 is a graph showing the relation between applied voltage and lighttransmittance for several representative wavelengths in liquid crystaldevices used in a specific example of the present invention and acomparison example;

FIG. 6 is a graph showing the relation between the transmittance oflight having a wavelength of 600 nm and the temperature of a liquidcrystal device used in a specific example of the present invention;

FIG. 7 is a graph showing the relation between the responsiveness of aliquid crystal device and time in a temperature environment of 100° C.;

FIG. 8 is a graph showing the relation between the transmittance and thespectrum of transmitted light for different duty ratios of appliedvoltages in a liquid crystal apparatus in Specific Example 1;

FIG. 9 is a graph showing the relation between the transmittance and thespectrum of transmitted light for different applied voltages in theliquid crystal apparatus of Comparison Example 1;

FIG. 10 is a schematic view for illustrating a method of measuringilluminance and color temperature in the illumination apparatuses ofSpecific Example 2 and Comparison Example 2;

FIG. 11 is a graph showing the relation between the duty ratio ofapplied voltage and the illuminance of projected light in theillumination apparatus of Specific Example 2;

FIG. 12 is a graph showing the relation between the illuminance ofprojected light and the color temperature in the illumination apparatusof Specific Example 2;

FIG. 13 is a graph showing the relation between the voltage value ofapplied voltage and the illuminance of projected light in theillumination apparatus of Comparison Example 2; and

FIG. 14 is a graph showing the relation between the illuminance ofprojected light and the color temperature in the illumination apparatusof Comparison Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND OF THE BEST MODEOF THE INVENTION

Now, the present invention will be described in conjunction with thedrawings illustrating one embodiment of a liquid crystal apparatus andan illumination apparatus incorporating the liquid crystal apparatus.

As illustrated in FIG. 1A, the liquid crystal apparatus of the presentembodiment is formed of a liquid crystal device L, and a driving circuitD which controllably drives this liquid crystal device L.

Further, the illumination apparatus of the embodiment as illustrated inFIG. 1B includes a light source lamp P, a lens S, and a housing orcontainer C for accommodating them, and the liquid crystal device Ldisposed in the optical path of light projected from light source lamp Pas a light adjusting element. Note that in the above-describedillumination apparatus, in order to protect liquid crystal device L fromultraviolet rays and infrared rays emitted from light source lamp P, adielectric multilayer film that reflects these rays may be providedbetween the light source lamp P and the liquid crystal device L, or sucha dielectric multilayer film may be disposed on a surface of the liquidcrystal device L on the side facing the light source lamp P. Further, inorder to increase the extinction ratio, a plurality of such liquidcrystal devices L may be stacked or arranged adjacent one another.

Liquid crystal device L is formed as illustrated in FIG. 2 bysandwiching a composite film 1 between a pair of transparent substrates20, 20. However, a transparent conductive film 21 is formed on thesurface of transparent substrate 20 in contact with the composite film1.

Glass, plastic film such as polyethyleneterephthalate (PET) andpolyethersulfone (PES) or the like is used for the transparent substrate20, while a transparent conductive film 21 may be provided by forming aconductive film such as ITO (Indium Tin Oxide) and SnO2 on the surfaceof transparent substrate 20 by means of deposition, sputtering orcoating.

The composite film 1 may be any one of various composite films capableof switching between two stages of optical states, opaque andtransparent states, depending upon the state of the electrical inputfrom the above-described transparent conductive films 21, 21. Aparticular embodiment is illustrated in FIG. 3A, wherein composite film1 includes a liquid crystal material 12 filled into a series of pores ina film that forms a transparent matrix 11 having a sponge-likestructure. FIG. 3B shows an alternative embodiment of the composite film1, in which liquid crystal material 12 is dispersed in particles in afilm that forms a transparent matrix 11. The composite films of thesestructures are formed, for example, according to any one of thefollowing three methods, which each use transparent substrates 20 thatinclude transparent conductive films 21 that have been respectivelyformed on a surface of each substrate 20.

i) When the transparent matrix is formed of a polymer, for example, aliquid having the polymer and a liquid crystal material dissolved ordispersed in a suitable solvent is applied onto the surface of atransparent conductive film 21 of a transparent substrate 20. Then, theso-called solvent evaporation method is carried out to evaporate thesolvent and separate the polymer and the liquid crystal material fromeach other, so as to form the composite film. Then, the othertransparent substrate 20 is joined onto the surface of the thus formedcomposite film so that its transparent conductive film 21 comes intocontact with the composite film, whereby the liquid crystal devicehaving the layered structure as illustrated in FIG. 2 is completed.

ii) In a suspension method, a milky mixture solution of a hydrophilicpolymer such as polyvinyl alcohol and a liquid crystal material isapplied onto the surface of the transparent conductive film 21 of onetransparent substrate 20, water in the solution is evaporated, and theliquid crystal material is dispersed in the polymer in particles to forma composite film. Then, the other transparent substrate 20 is joinedonto the surface of the thus formed composite film so that itstransparent conductive film 21 is in contact with the composite film,whereby the liquid crystal device having the layered structure as shownin FIG. 2 is completed.

iii) Further, in a polymerization-induced phase separation method, amixture solution of polymer precursor (prepolymer), a liquid crystalmaterial, and a polymerization initiator is injected between thetransparent conductive films 21 of two transparent substrates 20, andthen polymerization and cross-linking reactions are induced byultraviolet rays or heat, whereby the polymer and the liquid crystalmaterial are separated from each other. Thus, a composite film havingthe liquid crystal material dispersed in the polymer matrix is formed,and the liquid crystal device having the layered structure shown in FIG.2 is completed.

The thickness of such a composite film must be at least as large as thewavelength of visible light in order to use a liquid crystal deviceincluding the composite film in a system based on controlled lightscattering. However, a thickness that is too large excessively increasesthe driving voltage of the element, and therefore a thickness in therange from about 10 to 30 μm is appropriate in practice.

The liquid crystal material for forming the composite film is notspecifically limited, but the use of such a material having a largerefractive index anisotropy (Δn) and a large dielectric anisotropy (Δε)is preferred for obtaining excellent characteristics. Further, theliquid crystal material may be any of the materials demonstratingvarious conventionally known liquid crystal phases such as a nematicliquid crystal phase, a smectic liquid crystal phase, and achiralnematic liquid crystal phase. For the chiralnematic liquid crystalphase, a cholesteric liquid crystal as well as a mixture of usualnematic liquid crystal and a chiral component such as theabove-described cholesteric liquid crystal may be used. Further, inorder to provide the liquid crystal with a coloring function, variousconventionally known dichroic pigments may be mixed into the liquidcrystal.

However, the above-described liquid crystal material is preferably amaterial having high speed responsiveness that will permit changing froman opaque state to a transparent state or vice versa in response to apulse-shaped voltage applied to the composite film 1 according to amethod of driving a liquid crystal device according to the presentinvention, which will be described below.

A polymer is mainly used as the transparent matrix 11, i.e. the filmmaterial, which, together with the above-described liquid crystalmaterial 12, constitutes the composite film 1. The polymer is preferablyone with a high transparency to visible light, for example methacrylicpolymer represented by PMMA, epoxy urethane resin or the like. Note thatthe transparent matrix is not limited to polymers, and may instead beformed of a transparent inorganic material such as glass, or of apolymer having such an inorganic material dispersed therein.

In composite film 1 of the above-described structure, the relation setforth in FIG. 5 is established between the transmittance and the appliedvoltage. More specifically, when no voltage is applied, the liquidcrystal molecules are in a random state restricted by the anchoringeffect from the transparent matrix, and therefore the composite film 1is in an opaque state with a low transmittance. However, when aconventional voltage of a positive-negative alternating rectangularwaveform, for example as illustrated in FIG. 4B, is applied to thetransparent conductive films 21, the liquid crystal molecules that havea positive dielectric anisotropy (Δε) are oriented in the direction ofthe electric field depending on the amplitude of the applied voltage (V₂in FIG. 4B). When the amplitude of the voltage increases, thedisturbance of the orientation decreases, and the transmittanceincreases. If a voltage with a sufficient amplitude is applied to thecomposite film 1, the film shows a transparent state.

Driving circuit D is used to drive liquid crystal device L operating inaccordance with the above-described principle by the driving methodaccording to the invention. To achieve this as illustrated in FIG. 1A,the driving circuit D is electrically connected to the transparentconductive films 21, 21 of the pair of transparent substrates 20, 20 ofliquid crystal device L.

As illustrated by an example in FIG. 4A, the driving voltage appliedacross the region between the transparent conductive films 21, 21 of theliquid crystal device L by driving circuit D is, according to theinvention, a pulse-shaped voltage alternately switching between a firstvoltage state in which the composite film 1 is brought into an opaquestate (0 V in the figure), and a second voltage state sufficient forbringing the composite film 1 into the final transparent state (thestate of the applied rectangular waveform voltage alternating positiveand negative at ±V_(T)). When such a pulse-shaped driving voltage isapplied, the composite film 1 responds by alternately repeating theopaque state and the transparent state, and the ratio of the two statesper unit time period determines the power of the non-scattered lighttransmitted through the composite film 1 per unit time period.

Accordingly, the power of the non-scattered light transmitted throughthe composite film 1 per unit time period can be continuously adjustedbetween the opaque state and the transparent state by inputting anexternal signal to driving circuit D, which controls the time period ofthe second voltage state, i.e. the applied voltage state. The ratio ofthe time period T_(on) of the second voltage state to the total of thetime period T_(on) and the time period T_(off) of the first voltagestate, i.e. zero voltage, is referred to as the duty ratio and is givenby the following expression:

    T.sub.on /(T.sub.on +T.sub.off)=duty ratio.

At that time, the spectrum distribution of the transmitted light issubstantially identical to the transmittance spectrum in a substantiallytransparent state. Thus there is no possibility that the spectrumdistribution of the transmitted light will shift and that the color toneof the transmitted light will abnormally change depending upon theapplied voltage, particularly in the middle state between the opaquestate and the transparent state. Therefore the spectrum distributionbecomes substantially constant.

A flicker problem could arise when a liquid crystal apparatus accordingto the invention is used for illumination control for television ormovie filming, or in the light source of a projecting type televisionreceiver, a display, an indoor/outdoor illumination apparatus or otherillumination devices. In such applications, flickers are sometimesgenerated if the frequency of transition between the opaque andtransparent states of the composite film 1 is different from the framefrequency of the above described television apparatus or the frequencyof the light radiated from the above-described light source apparatus.In order to effectively prevent such a disadvantage, the frequency f_(A)represented as 1/(T_(on) +T_(off)) from said time periods (T_(on),T_(off)) is set to match the frame frequency f_(C) of the image sensingapparatus such as the above-described television apparatus, or thefrequency f_(B) of the light radiated from the light source apparatus,or an integer multiple of f_(C) or f_(B). As a further alternative thedifference between an integer multiple of f_(C) and an integer multipleof f_(B) is set higher than the limit frequency of the flickers, i.e.the repeating number of cycles per second at the limit where theflickers are not noticeable.

In order that the composite film 1 repeats the opaque state and thetransparent state in response to the high frequency pulsed alternatingvoltage as described above, the time (τ_(on)) required for switchingfrom the opaque state to the transparent state and the time (τ_(off))required for switching from the transparent state to the opaque stateshould be shorter than the above described T_(on) and T_(off),respectively. Among the above mentioned times, τ_(on) depends largely onthe intensity of the applied electric field, and it decreases as theelectric field intensity increases. Therefore τ_(on) can be made shorterthan T_(on) by increasing the intensity of the electric field applied tocomposite film 1.

Meanwhile, since τ_(off) is less dependent on the electric fieldintensity, another measure must be considered in order to make τ_(off)shorter than T_(off). Various measures are effective for shorteningτ_(off), for example: (1) to use a liquid crystal material with a smallviscosity and a large elasticity constant, (2) to enhance the anchoringforce of the liquid crystal transparent matrix, (3) to reduce the meshsize of the transparent matrix constituting the film, (4) to use aliquid crystal material whose polarity of dielectric anisotropy changeswith the frequency, and (5) to use a cholesteric liquid crystal materialas disclosed in Japanese Patent Laying-Open No. 4-119320.

Further, since the viscosity of the liquid crystal material is reducedas the temperature of the element increases, and thereby both τ_(on) andτ_(off) are shortened, using the element at a high temperature isanother effective method. In an illumination apparatus according to theinvention, a liquid crystal device L is disposed in the optical path oflight projected from a light source lamp P. Therefore the element can beheated to a temperature of 100° C. or higher depending upon theintensity of the light beam passing through the liquid crystal device.In this case, it is necessary for the liquid crystal device L to operatewithin a temperature range of 100° C. or higher to continue operatingnormally.

To this end, a liquid crystal material having a liquid crystal phasewithin a temperature range of at least 100° C. or higher, and preferably150° C. or higher, should be used. A liquid crystal phase such as anematic phase and a cholesteric phase is preferable in this case in viewof the response time. Although the smectic phase liquid crystal isgenerally low in response time as compared to the liquid crystals withthe above-described two phases, the smectic phase liquid crystal may beused in applications relying on its memory property. Material propertiesof a liquid crystal such as refractive index anisotropy, dielectricanisotropy, viscosity, elasticity constant, and pitch length in thecholesteric phase generally vary with temperature. Thus, it is desirableto select and use a material having suitable material properties at atemperature of 100° C. or higher, taking into account the change of therefractive index of the transparent matrix or the like. Further, when aninorganic material having high-heat resistance, such as glass or apolymer material, is used for the transparent matrix, it is effective touse such a material having a glass transition temperature of 100° C. orhigher, preferably 150° C. or higher, or a cross-linked polymermaterial.

In order to equalize the respective spectrum distributions of the opaquestate and the transparent state, it is appropriate to control the stateof dispersion of liquid crystal in the composite film. The spectrum inthe transparent state is generally a spectrum near white, for example,as represented by a spectrum at an applied voltage of 100 V in FIG. 8.The composite film shows an opaque state when no voltage or a voltage ofa low value is applied, and the opaque state varies depending upon thestate of dispersion of the liquid crystal filling, in other words theshape and size of the series of pores or particles. If, for example, thegrain or pore size is uniform, a selective scattering phenomenon inwhich light of a certain wavelength is particularly strongly scatteredis sometimes encountered, which impairs the whiteness of thenon-scattered light and is therefore not preferable. It would bepreferable that the structure of particles or pores is appropriatelyuneven or the particles or pores are so sized that such selectivescattering will not occur in the wavelength band of visible light.

The shape of serial pores or particles can be controlled by controllingmanufacturing conditions. Since the size or shape of pores can becontrolled, for example, by controlling the solvent evaporation rate inthe evaporation separation method as described above for an exampleembodiment, the respective spectrum distributions of the opaque stateand the transparent state can substantially be equalized, by optimizingthe temperature, the atmospheric pressure, the kind of solvent, and thecomposition ratio of a mixture solution used when manufacturing theliquid crystal device. A method for optimization as such can beappropriately selected depending upon the particular manufacturingmethod used to prepare the composite film.

When a liquid crystal apparatus having the above described structure isused for a liquid crystal display or various illumination apparatus, forexample, it is possible to divide composite film 1 into a plurality ofsegments that can be individually driven according to any desireddisplay pattern. To achieve this, the transparent conductive film 21 oftransparent substrate 20 is patterned into a shape corresponding to theabove described segments, and driving circuit D is connected to thesegments of the film 21 so as to apply a separate driving voltage forevery segment.

According to the method of driving a liquid crystal device according tothe invention described above, the spectrum distribution of transmittedlight can be kept substantially constant, and the transmittance of lightthrough the liquid crystal device can be sequentially, i.e. continuouslyvariably controlled.

Accordingly, when a liquid crystal apparatus according to the inventionemploying this driving method is used for a light-adjusting window or adisplay apparatus, for example, the color does not shift in a middletransmittance state, yet the transmittance in the case of thelight-adjusting window and the display density in the case of thedisplay device can be sequentially and freely adjusted. Suchadjustability is a significant function and not found with conventionaltechniques.

An illumination apparatus according to the present inventionincorporating the above-described liquid crystal apparatus cansequentially and freely control the power of projected light in acontinuously variable manner without changing the spectrum of theprojected light. The present apparatus can therefore be used for variousillumination devices, such as indoor illumination devices, andillumination devices for television or movie filming or photographing,or for an illumination apparatus having a light-adjusting function in aprojecting type television receiver, projector, or slide projector.

An example of manufacturing a liquid crystal device will now bedescribed. 70 weight portion of a nematic liquid crystal material (phasetransition temperature: 15° C. for crystal layer to nematic phasetransition, 170° C. for nematic layer to isotropic phase transition;manufactured by Merk Japan), 25 weight portion of an acrylic polymermaterial, and 5 weight portion of a cross-linking agent (polyisocyanate;available from Takeda Yakuhin Kogyo, No. A-3) were dissolved intodichloromethane as a solvent to prepare an application liquid with asolute concentration of 20%. The above-described acrylic polymermaterial is a copolymer of acrylic acid ester containing 20% by weighthydroxyethylmethacrylate as a component, and is cross-linked by areaction of a terminal OH group of hydroxyethylmethacrylate and thecross-linking agent.

Then, the application liquid was applied onto a glass substrate, onwhich a transparent conductive film had been formed, by a bar coatmethod, the solvent was evaporated in air at 1 atm. whereby a compositefilm as thick as 17 μm was formed, and then the solvent remaining in thecomposite film was removed by heating to a temperature of 100° C.Another glass substrate having a transparent conductive film identicalto the above-mentioned glass substrate was joined onto and pressed intoclose contact with the composite film under a pressure of about 1kgf/cm² to manufacture a liquid crystal device.

When a stepped voltage of 150 V was applied between the pair oftransparent conductive films of the above described liquid crystaldevice, the time elapsed until the transmittance reached 90% of asaturation transmittance was measured as the response time τ_(on) forchanging from the opaque state to the transparent state. Then, with theapplied voltage at 0 V, the time elapsed until the transmittanceattenuated to 10% of the saturation transmittance was measured as theresponse time τ_(off) for changing from the transparent state to theopaque state. The results were τ_(on) =0.2 msec, τ_(off) =0.9 msec.

Then the above-described liquid crystal device was set in aspectrophotometer (Model No. UV-160 manufactured by Shimazu Seisakusho),and the relation between the applied voltage and the transmittance oflight having wavelengths of 400 nm, 500 nm, 600 nm, and 700 nm wasmeasured by applying a rectangular waveform voltage of 200 Hz across theregion between the two transparent conductive films of the transparentsubstrates in successively increasing 2 V steps starting from 0 V. Theobtained result is set forth graphically in FIG. 5. The ordinate in FIG.5 indicates the transmittance of the device in terms of % normalized fordifferent wavelengths, while the abscissa shows the amplitude of therectangular waveform voltage.

As shown in FIG. 5, the transmittance of light through theabove-described liquid crystal device was below 10% at an appliedvoltage of 10 V or lower for all wavelengths of light. At an appliedvoltage of 80 V or higher, the transmittance of light of a wavelength of500 nm and greater was around 90% of the saturation transmittance of therespective wavelength, in other words a transparent state existed.Further, the relative magnitude of the transmittance of light having theabove-mentioned wavelengths changed with voltage, whereby the color toneof the above-described liquid crystal device driven in the abovedescribed manner changed in response to the effective value of theapplied alternating voltage.

Furthermore, the transmittance of light of 600 nm wavelength through theliquid crystal device was measured at applied voltages of 0 V and 100 Vwhile the temperature of the element was changed between 30° C. and 150°C. As a result, it was found that the responsiveness for changingbetween opaqueness and transparency in response to the applied voltagewas maintained even in the temperature range in excess of 100° C. and upto about 140° C., as shown in FIG. 6.

Further, when the liquid crystal device was placed in a temperatureenvironment of 100° C. and the change in responsiveness was examined, asillustrated in FIG. 7, it was found that the respective transmittance atrespective applied voltages of 0 V and 100 V did not change after thepassage of 1000 hours. Thus, it was found that the element continued tobe responsive even after long times at high temperatures.

SPECIFIC EXAMPLE 1

The above-described liquid crystal device was connected to a drivingcircuit to make a liquid crystal apparatus as shown in FIG. 1A. Then,the liquid crystal device of the liquid crystal apparatus was set intothe spectrophotometer in the same manner as above, and a pulse-shapeddriving voltage as shown in FIG. 4A (voltage value V₁ =150 V, T_(on)+T_(off) =8.33 msec, frequency 120 Hz) was applied by the drivingcircuit. The duty ratio [=T_(on) /(T_(on) +T_(off))] of the appliedvoltage was changed in a stepped manner ranging from 0/16 to 16/16 insteps of 2/16. The measurement result of the change of the spectrum oftransmitted light is set forth in FIG. 8.

From the result shown in FIG. 8, it was confirmed that the transmittancechanges as the duty ratio is changed, but the form of the spectrumdistribution of light at each transmittance hardly changes. Thereforethe transmittance of light through the liquid crystal device can beadjusted while keeping the spectrum distribution almost constant, usingthe liquid crystal apparatus of Specific Example 1.

COMPARISON EXAMPLE 1

The above described liquid crystal device was connected to a drivingcircuit that generated the rectangular waveform voltage shown in FIG. 4Bto make a liquid crystal apparatus of Comparison Example 1. Then, theliquid crystal device of the liquid crystal apparatus was set into thesame spectrophotometer as above, and a rectangular waveform voltage (T₁=T₂ =2.5 msec, frequency 200 Hz) as shown in FIG. 4B was applied by thedriving circuit. The voltage value V₂ of the applied voltage was changedto levels shown in FIG. 9. The result of measurement of the change ofthe spectrum of transmitted light is set forth in FIG. 9.

From the result shown in FIG. 9, it was found that not only does thetransmittance change as the voltage is changed, but also the form of thespectrum of light at each transmittance greatly changes.

SPECIFIC EXAMPLE 2

The liquid crystal apparatus of the above Specific Example 1 wascombined with a tungsten lamp (300 W) as a light source and a lens tomake an illumination apparatus as shown in FIG. 1B. With the lamp lit,the temperature of the liquid crystal device was measured, using aradiation thermometer, as about 100° C.

Then, a chrominance colorimeter M was disposed in front of theillumination apparatus in the optical path of light projected from thelight source lamp, as shown in FIG. 10. Then, a pulse-shaped drivingvoltage the same as in the above Specific Example 1 was applied to theliquid crystal device and the duty ratio of the applied voltage waschanged in a stepped manner. The illuminance and color temperature weremeasured at a measuring point in the optical path of the transmittedlight at a distance d=lm away from the liquid crystal device L usingchrominance colorimeter M. The resultant relation between the duty ratioof the driving voltage and the illuminance is set forth in FIG. 11 andthe relation between the illuminance and the color temperature is shownin FIG. 12.

As shown in FIG. 11, the duty ratio of the driving voltage and theilluminance were approximately in a proportional relation to each other,which demonstrated that the illuminance of the projected light can becontrolled by changing the duty ratio of the driving voltage in theillumination apparatus of Specific Example 2. Further, as shown in FIG.12, the color temperature of the projected light hardly changed even ifthe illuminance was changed. Thereby it was determined that the power ofthe projected light can be controlled without changing the spectrum ofthe projected light in the illumination apparatus of Specific Example 2.

COMPARISON EXAMPLE 2

The liquid crystal apparatus of the above Comparison Example 1 wascombined with a tungsten lamp (300 W) as a light source and a lens tomake an illumination apparatus. A chrominance colorimeter M was disposedin the optical path of the light projected from the light source lamp infront of the illumination apparatus. Then, a rectangular waveformvoltage the same as in the above Comparison Example 1 was applied to theliquid crystal device and its voltage value V₂ was varied or changed,and the illuminance and color temperature were measured at the samemeasuring point as for Specific Example 2 using chrominance colorimeterM. The relation between the voltage value of the applied voltage and theilluminance is set forth in FIG. 13, and the relation between theilluminance and the color temperature is set forth in FIG. 14.

As shown in FIG. 13, it was found that the illuminance of the projectedlight can be controlled by changing the voltage value of the appliedvoltage, but the spectrum of the projected light changes as well, asunderstood from FIG. 14.

As has been described in detail above, the liquid crystal deviceaccording to the invention continues to be responsive in a hightemperature environment of 100° C. or higher. Also, by driving a liquidcrystal device according to a method of the invention, the transmittanceof light through the liquid crystal device can be controlled whilekeeping the spectrum distribution of the transmitted light almostconstant. When the liquid crystal apparatus according to the inventionemploying the above-described driving method is used for alight-adjusting window or a display, for example, no color shift occursin the state of intermediate transmittance, and yet the transmittance ofthe light-adjusting window or the density of the display can becontinuously and freely adjusted. Furthermore, in the illuminationapparatus of the present invention incorporating the above-describedliquid crystal apparatus, the power of the projected light can becontinuously and freely controlled without changing the spectrum of theprojected light. Accordingly, the illumination apparatus of theinvention can be used for various illumination devices, indoorillumination devices, illumination devices for television or moviefilming or photographing, or alternatively for a projecting typetelevision receiver, a projector, a slide projector or the like.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated that it is intended to coverall modifications and equivalents within the scope of the appendedclaims.

What is claimed is:
 1. An illumination apparatus, comprising:a lightsource; a liquid crystal device disposed in an optical path of lightprojected from said light source and having a composite film including aliquid crystal material and a film forming a transparent matrixreceiving said liquid crystal material, and having substrates includinga pair of transparent conductive films sandwiching said composite filmtherebetween; and a driving circuit electrically connected to saidconductive films and applying thereto an alternating voltage waveformalternately switching between first and second voltage states in whichsaid composite film respectively attains an opaque state and atransparent state, wherein the ratio of the time period (T_(off)) of theopaque state to the time period (T_(on)) of the transparent state of thecomposite film per unit time period is varied by pulse width modulationof said alternating voltage waveform for changing the time period ofboth voltage states with an externally applied signal, therebycontrolling the power of non-scattered light transmitted through saidcomposite film per unit time period without substantially changing thespectrum of the transmitted light; and wherein said composite film has anon-uniform distribution of said liquid crystal material therein so thatthe respective spectrum distributions of the opaque state and thetransparent state are substantially equal.
 2. The illumination apparatusas recited in claim 1, wherein the repeating frequency f_(A) of theopaque state and the transparent state induced in said liquid crystaldevice is set equal to a value selected from the group of valuesconsisting of the frequency f_(B) of light radiated from said lightsource, the frame frequency f_(C) of an image sensing apparatus usedwith said illumination apparatus, an integral multiple of f_(B), and anintegral multiple of F_(C).
 3. The illumination apparatus as recited inclaim 1, wherein said liquid crystal material in said liquid crystaldevice has a liquid crystal phase at a temperature of at least 100° C.and is responsive at said temperature to change from the opaque state tothe transparent state in response to said voltage waveform being appliedbetween said conductive films.
 4. The illumination apparatus as recitedin claim 3, for use in conjunction with an imaging device having a framefrequency f_(C), wherein the frequency f_(A) of the alternating voltagewaveform is set equal to an integral multiple of f_(C), and a responsetime τ_(on) for switching from the opaque state to the transparent stateis shorter than said time period T_(on), and a response time τ_(off) forswitching from the transparent state to the opaque state is shorter thansaid time period T_(off).
 5. The illumination apparatus as recited inclaim 1, wherein said film forming a transparent matrix is across-linked polymer material.
 6. The illumination apparatus as recitedin claim 5, wherein said cross-linked polymer is formed of an acrylicpolymer material containing polyisocyanate as a cross-linking agent. 7.The illumination apparatus as recited in claim 1, wherein the maximumvariation in the color temperature of said non-scattered lighttransmitted through said composite film is within about 300 K.
 8. Theillumination apparatus as recited in claim 1, wherein said compositefilm has a structure of said liquid crystal material filling pores insaid matrix film, and wherein said non-uniform distribution of saidliquid crystal material is provided in that said pores have at least oneof non-uniform sizes, non-uniform shapes, and a non-uniform distributionin said matrix film.
 9. The illumination apparatus as recited in claim8, wherein said matrix film has a three-dimensional network structureincluding a continuous interconnected series of said pores.
 10. Theillumination apparatus as recited in claim 9, wherein said networkstructure of said matrix film is formed by mixing a polymer and a liquidcrystal material in a suitable solvent and then evaporating said solventto separate said polymer and said liquid crystal material.
 11. Theillumination apparatus as recited in claim 10, wherein said polymer isselected from the group consisting of methacrylic polymer, epoxy resinand urethane resin.
 12. The illumination apparatus as recited in claim10, wherein said composite film includes 25 weight parts of said polymerand 70 weight parts of said liquid crystal material.
 13. Theillumination apparatus as recited in claim 10, wherein said solvent isdichloromethane.
 14. The illumination apparatus of claim 1, wherein saidcomposite film contains no dye.
 15. The illumination apparatus of claim1, wherein f_(A) defines the repeating frequency of the opaque state andthe transparent state induced in said liquid crystal device, f_(B)defines the frequency of light radiated from said light source, andf_(C) defines the frame frequency of an image sensing apparatus usedwith said illumination apparatus, and wherein f_(A) is so adjusted toobtain a condition selected from the group consisting of the conditionsthat the difference between f_(A) and f_(B) is not less than a criticalflicker frequency, and that the difference between f_(A) and f_(C) isnot less than a critical flicker frequency.
 16. The illuminationapparatus as recited in claim 1, wherein said composite film has astructure of independent particles of said liquid crystal materialdispersed within said matrix film, and wherein said non-uniformdistribution of said liquid crystal material is provided in that saidparticles have at least one of non-uniform sizes, non-uniform shapes,and a non-uniform distribution in said matrix film.
 17. An apparatus forcontrolling the brightness of light transmitted therethrough tointermediate brightness levels between a maximum opaque state and amaximum transmissive state while substantially maintaining in theintermediate brightness levels a nominal color spectrum of thetransmitted light in the transmissive state, wherein said apparatuscomprises a liquid crystal device that is arranged in the path of thelight and that includes a liquid crystal composite film sandwichedbetween two transparent conductive films, wherein said composite filmincludes a liquid crystal material non-uniformly dispersed within atransparent matrix film such that said composite film substantiallymaintains the nominal spectrum of the transmitted light also in themaximum opaque state, and said apparatus further comprises a drivingcircuit electrically connected to said conductive films and applyingacross said conductive films an alternating voltage waveform thatalternates between a first voltage state for a time T_(off) in whichsaid liquid crystal material is in the maximum opaque state and a secondvoltage state for a time T_(on) in which said liquid crystal material isin the maximum transmissive state, and that has the ratio of T_(on)/(T_(on) +T_(off)) adjusted to achieve a desired intermediatetime-averaged perceived brightness level of the light transmittedthrough said liquid crystal device without substantially altering thespectrum of the transmitted light from the nominal spectrum of thetransmitted light in the transmissive state.
 18. The apparatus of claim17, wherein said composite film contains no dye.
 19. The apparatus ofclaim 17, wherein said composite film consists essentially of saidliquid crystal material and said transparent matrix film.
 20. Theapparatus of claim 17, expressly excluding polarizers.
 21. The apparatusof claim 17, wherein the maximum variation of the color temperature ofthe spectrum of transmitted light among all of the intermediatebrightness levels is less than about 300 K.
 22. The apparatus of claim17, wherein said liquid crystal material has a liquid crystal phase at atemperature of at least 100° C. and remains responsive to switch betweenthe transmissive state and the opaque state at a temperature of at least100° C.
 23. A method of operating the apparatus of claim 17,comprising:(a) operating said driving circuit to apply said alternatingvoltage waveform across said conductive films, wherein said waveformalternates between said first voltage state for said time T_(off) andsaid second voltage state for said time T_(on), and (b) modulating apulse duration of at least one of said first voltage state and saidsecond voltage state to alter the ratio T_(on) /(T_(on) +T_(off)) andthereby to adjust the time-averaged power of light transmitted throughsaid composite film to the desired intermediate brightness level. 24.The method of claim 23, further comprising adjusting the repeatfrequency of said alternating voltage waveform to equal the frequency ofthe light or an integral multiple thereof.
 25. The method of claim 23,further comprising coupling said apparatus with an image sensing device,and adjusting the repeat frequency of said alternating voltage waveformto equal the frame frequency of the image sensing device or an integralmultiple thereof.
 26. The apparatus of claim 17, wherein said compositefilm has a structure of said liquid crystal material filling pores insaid matrix film, and wherein said non-uniform dispersion of said liquidcrystal material is provided in that said pores have at least one ofnon-uniform sizes, non-uniform shapes, and a non-uniform distribution insaid matrix film.
 27. The apparatus of claim 17, wherein said compositefilm has a structure of independent particles of said liquid crystalmaterial dispersed within said matrix film, and wherein said non-uniformdispersion of said liquid crystal material is provided in that saidliquid crystal particles have at least one of non-uniform sizes,non-uniform shapes, and a non-uniform distribution in said matrix film.